logging: classify bare rate-limit messages as transient

[ben-dz]

Summary of Changes

• Broaden dberror's rate-limit pattern from "rate limited" to "rate limit", so messages like rpcpool's HTTP 429 body — Connection rate limits exceeded, surfaced through the jsonrpc client's error dump where neither "rate limited" nor "status 429" substring-match — classify as ErrorTypeRateLimit/transient. The broader pattern also covers "rate limiting".
• Why it matters: transient classification selects the lenient escalation threshold (10 consecutive failures) instead of the strict one (3), so a throttled-but-progressing loop logs WARN rather than paging on-call. Observed during the 2026-07-23 testnet permission-events drain (indexer: drain permission-events accounts in durable chunks to break testnet poison loop #706): a mid-drain 429 from the DZ ledger RPC classified as Unknown and would have escalated as if actionable.

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	1	+3 / -1	+2
Tests	1	+6 / -0	+6

One pattern broadened, pinned by table-test cases.

Testing Verification

• New table cases (verified failing first) assert Classify → ErrorTypeRateLimit and IsTransient → true for the exact prod error text (jsonrpc dump format with Connection rate limits exceeded) and a "rate limiting" variant; existing cases confirm "rate limited" and "status 429" still classify unchanged.
• utils/... suite passes with -race.